Anti-Markovnikov Oxidation and Hydration of Terminal Olefins

Dalton Transactions

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Anti-Markovnikov oxidation and hydration of terminal oleﬁns Cite this: Dalton Trans., 2014, 43, 6952 Jiayi Guoa and Peili Teo*a,b

Received 22nd December 2013, Efficient syntheses of aldehydes and primary alcohols from terminal olefins are critical to the chemical Accepted 30th January 2014 laboratory and industry. Heavy emphasis has been placed on regioselective olefin functionalization in DOI: 10.1039/c3dt53600a recent decades. This perspective mainly focuses on advances in the Wacker-type oxidation and tandem www.rsc.org/dalton hydration of terminal olefins with anti-Markovnikov selectivity.

1. Introduction Another popular protocol is via alcohol oxidation but the Current methods for aldehyde production from carbon–carbon formation of primary alcohols can itself be a very diﬃcult multiple bonds are hydroformylation (eqn (1)) and alkyne process.4,5 The PdII-catalyzed oxidation of olefins to carbonyl Creative Commons Attribution 3.0 Unported Licence. hydration. However, hydroformylation produces the homolo- compounds, usually known as the Wacker oxidation, is one of gous aldehyde and anti-Markovnikov alkyne hydration often the most well-known Pd-mediated reactions and has been so – employs catalysts that are non-trivial to prepare.1 3 extensively adopted that terminal olefins are often viewed as masked ketones.6 The process involves the coordination of olefin to PdII followed by reaction of the η2-Pd-alkene complex ð1Þ with water. This reaction obeys Markovnikov’s rule in the majority of cases (Scheme 1).4 Nevertheless, aldehydes are – sometimes produced in specific cases.7 10 a This article is licensed under a Department of Chemistry, National University of Singapore, 3 Science Drive 3, Anti-Markovnikov hydration of olefins has been listed as S(117543), Singapore 11a b one of the top ten challenges of catalysis since 1993. Direct Institute of Chemical & Engineering Sciences, 1 Pesek Road, Jurong Island, – S(627833), Singapore. E-mail: [email protected]; Fax: +(65)-67791691; addition of water across a carbon carbon double bond is an Tel: +(65)-65161377

Open Access Article. Published on 30 January 2014. Downloaded 9/30/2021 8:55:31 PM. important industrial process for alcohol production. Linear

Jiayi Guo received her Ph.D in Peili Teo received her Ph.D in 2013 from Nanyang Technologi- 2008 from the National Univer- cal University, Singapore. Her sity of Singapore under the thesis focused on the synthesis supervision of Professor and characterization of bis-phos- T. S. Andy Hor as an A*STAR phorus stabilized metal com- Graduate Scholar. Her thesis plexes. In April 2013, she joined focused on coordination assem- the group of Assistant Professor blies of d8 and d10 metals with Peili Teo at the National Univer- N,O-mixed-donor ligands. Peili sity of Singapore, as a postdoc- then went on to work as a toral fellow. Her current research A*STAR postdoctoral research is focused on olefin functionali- fellow with Professor Robert Jiayi Guo zation via eﬃcient and selective Peili Teo H. Grubbs at California Institute catalysis systems. of Technology where she worked on Z-selective olefin metathesis catalysts and anti-Markovnikov olefi oxidation and hydration. Returning to Singapore in 2012, Peili began her independent research career as a scientist at the Institute of Chemical & Engineering Sciences, A*STAR, and as an assistant Professor at the National University of Singapore.

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2. Aldehyde-selective Wacker-type reactions of terminal oleﬁns

In 2006, an anti-Markovnikov Wacker reaction of styrene- related substrates was reported by Spencer et al.10 Reversal of the usual regioselectivity occurred stoichiometrically in the absence of reoxidants (eqn (4)) or catalytically, when heteropolyacid (HPA) was used as the terminal oxidant.

ð4Þ Scheme 1 General mechanism of Wacker oxidation.

The substrate scope of this optimised condition was studied for a large range of styrenes from electron-deficient to alcohols are commonly used in the flavoring, perfumery, lubri- electron-rich (Table 1). The usage of a heteropolyacid (HPA)

cant and cosmetic industries. For example, 1-dodecanol, com- containing molybdenum and vanadium H4[PMo11VO40]was monly known as lauryl alcohol, is commonly used as a reported to successfully achieve 75% yield of aldehyde in the

surfactant or emulsifier in detergents and personal care pro- presence of 12 mol% of PdCl2 (eqn (5)). With HPA, the process ducts.11b However, in accordance with Markovnikov’s rule,4 the is catalytic but only styrene and methylstyrene can be oxidized primary alcohols are difficult to obtain. Hydroboration/oxi- to the corresponding aldehyde in reasonable yields. dation is currently a popular indirect method to afford hydration products with anti-Markovnikov selectivity (eqn 4 ð Þ Creative Commons Attribution 3.0 Unported Licence. (2)). This process requires stoichiometric borane reagents that 5 are expensive and the boron waste generated is difficult to be recycled. Moreover, the usage of peroxides in the oxidation The proposed mechanism for aldehyde formation from step of hydroboration poses safety issues when the process is styrene-related substrates was also disclosed in this work adapted for large scale production. As a result, hydroboration (Scheme 2). The η4 nature of styrene acts as a pseudo-diene to has not been adopted in the industrial production of alcohols form the η3-palladium intermediate followed by nucleophilic from olefins yet. Hydroformylation/reduction is another two- attack. This η4-interaction between the palladium and sub- step method that can produce primary alcohols from olefins strate apparently contributes to the anti-Markovnikov albeit through a homologation process (eqn (3)).12,13 Thus, This article is licensed under a behaviour. there is a compelling need to develop more efficient catalysis Although moderate aldehyde selectivity was obtained in systems for anti-Markovnikov hydration of olefins. Spencer’s work, it required either stoichiometric heteropolyacid or stoichiometric palladium. Also, a limited substrate Open Access Article. Published on 30 January 2014. Downloaded 9/30/2021 8:55:31 PM. ð Þ 2 scope is compatible with this system. With linear olefins, Mar- kovnikov selectivity was obtained.

ð3Þ Table 1 Stoichiometric Wacker reaction of substituted styrenes under optimized conditions10 It is encouraging to note that some progress has been made recently from investigations on Wacker-type oxidation for alde- Ratio of R group aldehyde : ketone Yield % hyde production. By controlling the regioselectivity of the oxidation step, anti-Markovnikov hydration of olefins can be 4-OMe 10 : 1 63 realized through the subsequent reduction process in a H11:171 2-Me >25 : 1 89 tandem catalytic system. Despite this not being the most ideal 2,6-Di-Me 2 : 1 60 method in the direct hydration of an olefin, it is still a major Naphthyl 13 : 1 38 advancement in the development of selective catalysts for the 4-Me 7 : 1 61 4-Cl >25 : 1 76 anti-Markovnikov hydration of olefins. 4-CF3 >25 : 1 69 Much progress in aldehyde-selective Wacker oxidation and 4-tBu 9 : 1 82 anti-Markovnikov olefin hydration has been made since the 3-Me 5 : 1 50 3-Cl >25 : 1 80 last review on the former in 2007 by Muzart.6 In this perspec- 3-NO2 16 : 1 54 tive, we illustrate the most significant contributions to anti- 3,5-di-tBu 6 : 1 — Markovnikov olefin oxidation in recent years, especially in 2-OMe 17 : 1 95 2-F >25 : 1 78 areas of Wacker-like processes for olefin oxidation and tandem 2-Br >25 : 1 — hydration. 2,4-Di-Me >25 : 1 82

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Table 2 Catalyst screening for phthalimide-protected allylic amines8

Ratio Cat. Conversion % aldehyde : ketone

A 100 96 : 4 B 100 >99 : 1 C8094:6 Scheme 2 Proposed mechanism for anti-Markovnikov regioselectivity.

In 2007, the synthesis of aldehydes with moderate yields via Table 3 Substrate scope of oxidation of phthalimide-protected allylic Pd-catalyzed oxidation from functionalized olefins were sum- amines8 marized by Muzart.6 In his review, Wacker-type reactions involving successive intermediate steps of hydroxy-, alkoxy- and acetoxy-palladation were described. Most processes were restricted to activated olefins, that is, olefins containing a che- lating group located at an appropriate position, such as a hetero- atom or unsaturation, in order to result in the regioselectivity.

Creative Commons Attribution 3.0 Unported Licence. When the aldehyde-selective Wacker oxidation was employed Ratio towards terminal olefins of group-protected allylic amines, a Substrate Cat. aldehyde : ketone Yield % new catalytic methodology for the synthesis of β3-amino acids R=CH B >99 : 1 94 was reported by Feringa et al. in 2009.8 Phthalimide proved to 3 R=C5H11 B >99 : 1 91 be the optimal protecting group resulting in good conversion in all the methods A, B and C, investigated (Table 2). A >99 : 1 93 The substrate scope was also studied using methods A and B that gave full conversions (Table 3). It is noteworthy that sub- A94:674 strates with long alkyl chains could be oxidized with excellent This article is licensed under a yields and selectivities (R = C5H11). The high anti-Markovnikov selectivity was proposed to have resulted from coordination of the protecting group to the palladium species. R = Bn B >99 : 1 94

Open Access Article. Published on 30 January 2014. Downloaded 9/30/2021 8:55:31 PM. This new synthetic route to amino aldehydes provides an R = BnOCH2 B >99 : 1 93 alternative method to prepare amino acids or amino alcohols R = Ph B >99 : 1 95 via consecutive oxidation, reduction and deprotonation pro- – B >99 : 1 77 cedures (eqn (6)).14 16

A >1 : 99 89 ð6Þ

— (a) (i) NaBH , MeOH; R = Me 95%; (ii) H NH , EtOH, Δ;R= A 0 4 2 2 B — 0 Me 90%; (b) (i) 0.5% Mn-tmtacn (1,4,7-trimethyl-1,4,7-triaza-

cyclononane), Cl3CO2H, H2O2, MeCN; R = Me 87%; (ii) H2NH2, EtOH, Δ; R = Me 100%. The palladium-catalyzed anti-Markovnikov oxidation of allylic esters was also achieved by Feringa et al.17 In this Owing to rapid isomerization between allylic esters under system, excellent aldehyde selectivity could be obtained with the reaction conditions, a major advantage of this system is t catalyst loadings as low as 0.5 mol% Pd(II), in BuOH solvent that only the anti-Markovnikov product is aﬀorded in the oxi- under ambient conditions (Table 4). p-Benzoquinone was dation even when a mixture of branched and linear allylic employed as the oxidant. Most aliphatic allylic esters resulted esters was used (Scheme 3). in high aldehyde selectivity. The phenyl- and benzyl substrates This anti-Markovnikov oxidation of allylic esters provides a were oxidized with slightly lower yields. facile route to the synthesis of protected β-hydroxy aldehyde

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Table 4 PdII-catalyzed oxidation of branched allylic esters17

R Conv. A/M Yield %

H Full 11 : 1 78 CH3 Full 7 : 1 71 C2H5 Full 20 : 1 79 C5H11 Full 20 : 1 73 Scheme 4 Proposed mechanism for Pd-catalyzed allylic acetoxylation. 95% 20 : 1 45

Table 5 Aerobic allylic acetoxylation of terminal oleﬁns18 Full 20 : 1 71

Ph Full 20 : 1 52 Bn 95% 20 : 1 64

Substrate Time (h) E : Z Yield %

24 17 : 1 81 Creative Commons Attribution 3.0 Unported Licence. 24 14 : 1 79

Scheme 3 Oxidation of a mixture of linear and branched allylic esters. 24 6 : 1 68

with low Pd catalyst loadings from either the branched or linear allylic esters, or even a mixture of both. 48 10 : 1 76 PdII-catalyzed oxidation has also been adopted in allylic

This article is licensed under a – C H acetoxylation. In 2010, Stahl et al. demonstrated a 4,5-diaza- 24 19 : 1 70 fluorenone-ligated palladium (Fig. 1) catalytic strategy for the conversion of terminal alkenes to linear allylic acetoxylation

products under 1 atm of O2, eliminating the requirement for 24 21 : 1 84 Open Access Article. Published on 30 January 2014. Downloaded 9/30/2021 8:55:31 PM. the commonly-used oxidant 1,4-benzoquinone (BQ).18 In this work, three possible roles of BQ in the catalytic cycle 48 6 : 1 52 were reported (Scheme 4): (1) enhancement of nucleophilic attack from acetate on the π-allyl-PdII species (step II);19 (2) dis- 48 29 : 1 76 placement of the allylic acetate product in the Pd0 species upon C–O bond formation (step III);20 and (3) regeneration of Pd0 to PdII (step IV).21 It is envisioned that geometric 24 7 : 1 76 properties of the diazafluorenone ligand such as the bite angle, may destabilize PdII and facilitate C–O elimination from the π-allyl-PdII intermediate (step II), thus enabling the BQ-free 48 36 : 1 71 catalytic turnover. This PdII-catalyzed allylic acetoxylation was investigated with a number of alkenes under aerobic conditions (Table 5). 24 16 : 1 74

Nearly all the substrates examined resulted in good yields of the linear acetoxylation products. Furthermore, anti-Markovni-

kov hydration of α-olefins was achieved using an O2/H2- coupled process based on this PdII-catalyzed allylic acetoxyla- Fig. 1 4,5-Diazaﬂuorenone-ligated palladium catalyst. tion in a straightforward one-pot reaction (Scheme 5).

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Table 6 Palladium-catalyzed synthesis of terminal acetals from vinyl- arenes and pinacol23

Total yield of Substrate A and B (%)

73 Scheme 5 Net anti-Markovnikov hydration of terminal alkenes via a one-pot, three-step sequence. 79 This aerobic ligand-based strategy serves as an inspiration for future work on the role of ancillary ligands in PdII-catalyzed 76 oxidation and accordingly, omission of the use of undesirable oxidants. Lin et al. reported another example of aerobic Wacker- 83 type intramolecular aldol cyclization.22 Two frameworks of 2-naphthols and benzofurans were generated in acceptable II yields. This aerobic Pd -catalyzed oxidation is an attractive 64 method for the bioactive synthesis of natural products Creative Commons Attribution 3.0 Unported Licence. (Scheme 6). The steric demand of nucleophiles is considered as another 69 potential factor to influence the regioselectivity. In 2012, Kataoka et al. reported an anti-Markovnikov Pd-catalyzed oxidation mediated by the steric bulkiness of pinacol.23 This 39 process enables the synthesis of terminal acetals from vinyl- arenes, allyl aryl ethers, and 1,5-dienes in the presence of 71 10 mol% PdCl2(MeCN)2 and 2 equiv. benzoquinone as the oxidant (Table 6). The terminal acetals can undergo acid- This article is licensed under a catalysed hydrolysis to result in the corresponding aldehydes. The proposed mechanism for this acetalization is shown in 61 Scheme 7. Firstly η4-coordination occurred between styrene Open Access Article. Published on 30 January 2014. Downloaded 9/30/2021 8:55:31 PM. and the PdII species. The nucleophilic attack of the pinacol in 50 an anti-Markovnikov manner then aﬀorded the benzyl intermediate. Attack on the internal carbon would be unfavourable due to the steric repulsion between the phenyl group and 75 pinacol. The final acetal product was formed via β-hydride elimination followed by cyclization. The formation of a benzyl 84 intermediate in this process is considered to be the main factor leading to anti-Markovnikov selectivity. Although regioselectivity control by means of a bulky 68 solvent is eﬀective in Kataoka’s system, the acetalization method is still an indirect way of obtaining the aldehyde. Recently, a facile PdII-catalyzed process to convert olefins directly into aldehydes was achieved by Grubbs et al.24 By com- 48

bining PdCl2(MeCN)2, 1,4-benzoquinone (BQ) and t-BuOH in the presence of stoichiometric amounts of water, both high Scheme 6 One-pot aerobic Wacker-type synthetic route toward aldehyde selectivity and yields could be obtained with vinyl 2-naphthols and benzofurans. arenes (Table 7).

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Table 7 Oxidation reaction of substrates24

Substrate Aldehyde yield Selectivity

83 98

81 97

90 96 Scheme 7 The proposed reaction mechanism for acetalization.

42 98 Similar to the solvent eﬀect of Kataoka’s acetalization,23 the use of t-BuOH critically influences the regioselectivity in Grubbs’s system. Due to the bulkiness of t-BuOH, the linear vinyl ether is preferred, which constitutes the key factor for 96 99 high anti-Markovnikov selectivity. During the process, acid is

Creative Commons Attribution 3.0 Unported Licence. generated in the presence of water, upon which the vinyl ether is converted to aldehyde via acid-catalyzed hydrolysis (Fig. 2). 96 >99 This highly aldehyde-selective system shows a major improvement to current methods for Wacker oxidation. The by- product 1,4-hydroquinone (HBQ) in this reaction could be easily 74 99 converted to 1,4-benzoquinone (BQ) via aerobic oxidation.25,26 Moreover, its simplicity leads to more practical applications. More recently, Grubbs et al. also reported the aldehyde- selective Wacker-type oxidation of unbiased olefins using a 92 99 This article is licensed under a combination of catalytic tert-butyl nitrite with PdCl2(PhCN)2 27 and CuCl2 catalysts in the presence of oxygen. High anti- Markovnikov selectivity without reliance on substrate control 90 99

Open Access Article. Published on 30 January 2014. Downloaded 9/30/2021 8:55:31 PM. was achieved (Table 8). In this system, AgNO2 plays a key role in the significant improvement of selectivity and yield.

MeNO2 was found to be an important co-solvent. Good 59 96 functional-group tolerance was observed with the optimized conditions. 18 An O-labeling experiment was carried out and it was 72 99 observed that the oxygen atom was derived from the nitrite salt where the 18O was incorporated into the aldehyde in an 81% yield. Thus the mechanistic feature of this reaction was proposed to be a metal-mediated radical-type addition to

terminal olefins by NO2 species (Scheme 8). The success of this system provides a crucial lead for further development of catalytic anti-Markovnikov oxidation systems using unbiased aliphatic substrates. Later on, the authors further extended the study to the ketone synthesis from the more challenging internal olefins by Wacker-type oxidation.28 In fact, substrate control is an inherent challenge for Fig. 2 Proposed strategy for olefin oxidation. Wacker-type processes. Sigman and Werner made efforts to develop a useful catalyst-controlled oxidation method which is not affected by functional groups on olefins or steric effects of line) catalyst formed in situ. It is believed that the electron-poor solvents.29 The study revealed a TBHP (tert-butylhydroperoxide)- quinoline module of the ligand facilitates olefin coordination mediated oxidation arising from a Pd-quinox (quinoline oxazo- (Scheme 9). Markovnikov selectivity is achieved here.

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Table 8 Aldehyde-selective Wacker-type oxidation of unbiased oleﬁns27

Yield of Substrate aldehyde Sel. Scheme 9 TBHP-mediated Wacker oxidation using the Pd-quinox 63 79 catalyst.

61 79 Table 9 Aerobic oxidation of 1-alkene30

70 89 59 79

51 67

Substrate Time (h) Yield (%) 65 82 4.5 92 59 81

Creative Commons Attribution 3.0 Unported Licence. 787 60 80

69 89 587

64 90 489

This article is licensed under a 793

584 Open Access Article. Published on 30 January 2014. Downloaded 9/30/2021 8:55:31 PM.

581

771

Scheme 8 18O-labeling experiment and radical model. 769

781 The use of an electronically asymmetric ligand in this Wacker oxidation has been shown to critically aﬀect the environment of the metal center. Thus ligand design is crucial 771 to achieve catalyst control for Wacker-type process and provides insight to realizing more challenging anti-Markovnikov functionalization. Apart from the widely known Wacker-type methods for 573 synthesizing aldehydes from olefins, Che et al. reported a ruthenium-porphyrin-catalyzed aldehyde synthesis in 2008 774 (Table 9)30. It was the first example of an aerobic epoxidation– isomerization (E–I) reaction (Scheme 10), forming aldehydes

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Scheme 10 Ruthenium porphyrin-catalyzed oxidation of 1-alkenes to aldehyde (E–I reaction).

Scheme 12 Proposed catalytic cycle for the hydration of terminal ﬁ VI ole ns to primary alcohols. Fig. 3 Ruthenium porphyrin catalyst [Ru (tmp)Cl2].

The proposed mechanism involves the formation of a hydrido-Pt(II)-hydroxo complex, followed by protonation to form an aqua complex and subsequent coordination of the olefin to platinum. The hydroxide anion then attacks the co-

Creative Commons Attribution 3.0 Unported Licence. ordinated olefin to form a Pt-alkyl complex which then undergoes trans–cis isomerization resulting in the alkyl and hydride Scheme 11 Fe-catalyzed oxidation of terminal olefins. moiety being cis to one another. The complex then undergoes C–H reductive elimination to yield the primary alcohol (Scheme 12). in up to 93% yields, using the ruthenium porphyrin catalyst Despite the elegance of Trogler’s method, the system was VI [Ru (tmp)Cl2] (Fig. 3). subsequently claimed to be irreproducible by Ramprasad Lahiri et al. reported an iron catalyzed aldehyde-selective et al.33 It was also speculated by Grushin et al.34 that the oxidation of terminal olefins using Fe(BF4)2·6H2O with the observed reactivity in Trogler’s system may be due to some This article is licensed under a commercially available ligand dipic (pyridine-2,6-dicarboxylic impurity present in the reaction mixture. 35 acid) and PhIO as an oxidant in CHCl3 at room temperature In common with Grushin, Parkins et al. carried out tests 31 (Scheme 11). This catalytic system functions well with the on the platinum catalysts used in Trogler’s process. In con- inexpensive and simple iron catalyst under mild conditions. Open Access Article. Published on 30 January 2014. Downloaded 9/30/2021 8:55:31 PM. sideration of the unusual method adopted by Trogler for the ’ ’ In summary, Che s ruthenium- and Lahiri s iron-catalyzed preparation of the catalyst precursor trans-[(Me3P)2PtHCl], systems provide unique and efficient methods for aldehyde Parkins et al. tested the catalytic activity of dinuclear platinum 36 synthesis from terminal olefins, which significantly comp- complexes [(dppe)Pt(μ-H)2-PtH(dppe)]BF4, [(Me3P)2Pt(μ-H)2- 37 lements the conventional Wacker-type oxidation. PtH(PMe3)2]Cl and [(Et3P)2Pt(μ-H)2PtH(PEt3)2]PF6, with sodium hydroxide, water and 1-octene in either THF or methanol (Fig. 4). Although these conditions corresponded most 3. Tandem anti-Markovnikov closely to that reported by Trogler, no 1-octanol was detected. fi Furthermore, during the investigation of hydration of hydration from terminal ole ns 38 1-octene using [Ph(PPh3)Pt(μ-H)(µ-PPh2)Pt(PPh3)2]BF4 (Fig. 5), Anti-Markovnikov hydration of terminal olefins remains a an impurity identified as oct-1-ene-3-hydroperoxide, was long-standing challenge to organic chemists.11 Studies on observed in the acidic methanolic solution of 1-octene. The direct and efficient catalytic processes have attracted much impurity underwent a Hock rearrangement and 1-methoxy-1- attention in the last few decades. In 1986, Trogler and Jensen vinyloxyhexane was formed as shown in Scheme 13.39 reported the first example of Pt-catalyzed regioselective hydration of non-activated terminal alkenes to primary alcohols.32 The direct transformation of 1-hexene to 1-hexanol is catalyzed by a species formed from the reaction between trans-

PtHCl(PMe3)2 and NaOH in a water and 1-hexene mixture, containing a phase-transfer catalyst, NEt3(CH2Ph)Cl. Hydration of 1-dodecene to 1-dodecanol by the same procedure occurs at Fig. 4 Dinuclear platinum complexes employed by Parkins and Richard 100 °C. for octene hydration.

Perspective Dalton Transactions

Fig. 5 Platinum complex [Ph(PPh3)Pt(μ-H)(PPh2)Pt(PPh3)2]BF4.

Scheme 13 Reaction mechanism showing the Hock rearrangement of oct-1-ene-3-hydroperoxide in acidic methanol.

Scheme 14 Proposed mechanism for the formation of 1-methoxy- Fig. 6 Parkin’s trinuclear platinum cluster. octane using trinuclear platinum catalyst. Creative Commons Attribution 3.0 Unported Licence. It was suspected that Trogler’s catalytic system32 might have contained some trinuclear species. The trinuclear platinum 40 Table 10 Phosphine-catalyzed hydration and hydroalkoxylation41 cluster [(dppe)3Pt3H3]+ (Fig. 6) was also tested by Parkins et al. for hydration activity. 1-Octanol was still not detected using this trinuclear platinum cluster as the hydration catalyst. However, the anti- Markovnikov product 1-methoxyoctane was observed in the product mixture. This suggests that a reaction between the This article is licensed under a hydroperoxide impurities in 1-octene and the trinuclear plati- EWG R1 ROH Time (h) Yield % num cluster occurred, leading to the formation of 1-methoxy- COEt Me H2O2077 octane as the product instead. The proposed mechanism for MeOH 24 85

Open Access Article. Published on 30 January 2014. Downloaded 9/30/2021 8:55:31 PM. 1-methoxyoctane production from 1-octene is shown in 16 63 Scheme 14. COMe H MeOH 1 56 Due to the presence of varying amounts of hydroperoxide Me2CHOH 1 83 impurities in treated or untreated 1-octene, Parkins et al.’s PhOH 16 59 results were somewhat erratic. However, their studies shed Ph MeOH 72 0 some light on the problems of the platinum-catalyzed anti- CO2Me Me MeOH 36 71 Markovnikov hydration of terminal olefins initially reported by Trogler.32 CN H MeOH 4 79 In 2003, Toste et al. disclosed a base-catalyzed method for hydration and hydroalkoxylation of activated olefins using trialkylphosphine.41 Catalytic amounts of trimethylphosphine proved to be very active in catalyzing the hydration or hydroalkoxylation reaction, even in the absence of metals in their system. A variety of α,β-unsaturated substrates were tolerated under the reported conditions and only the highly conjugated 4-phenyl-3-buten-2-one was unreactive (Table 10). The proposed catalytic cycle is shown in Scheme 15. The phosphonium enolate is formed by nucleophilic attack from the phosphine to the unsaturated carbonyl compound, which spontaneously generates the alcohol by deprotonation. The alkoxide of the phosphonium ion pair then undergoes subsequent addition to give an enolate ion pair.42 Protonation of Scheme 15 Proposed catalytic cycle.

Dalton Transactions Perspective

Table 11 [Pd]/[Ru]-catalyzed hydration of functionalized styrenes, 1-octene and allylbenzene25

Substrate Product Yield % Selectivity Scheme 16 Proposed cooperative catalytic system for alcohol synthesis from oleﬁns and water. 84 ≥20 : 1

42 ≥20 : 1 the enolate leads to the formation of the β-alkoxy carbonyl and regeneration of the alkoxide with an ion pair to continue the cycle. This nucleophilic phosphine catalytic system can be 61 ≥20 : 1 used in the absence of additional transition metals, strong acids or bases but is limited to the αβ-unsaturated olefin class of substrates. 60 ≥20 : 1

Creative Commons Attribution 3.0 Unported Licence. Recently, a tandem catalyst system for the synthesis of primary alcohols from non-activated terminal olefins was reported by Grubbs et al.25 This strategy is based on PdII-cata- 72 ≥20 : 1 lyzed oxidation, acid-catalyzed hydrolysis and Ru-catalyzed reduction cycles (Scheme 16). This triple-relay system relies on three criteria, firstly, the oxidation step must be aldehyde- 75 ≥20 : 1 selective; secondly, the oxidation cycle must be compatible with the reduction cycle and thirdly, the migration of the hydride from Pd to Ru must be facile. 72 ≥20 : 1 This article is licensed under a In this catalytic system, t-BuOH was chosen as the solvent for PdII-catalyzed oxidation to result in aldehyde selectivity.43 A transfer hydrogenation catalyst, Shvo’s complex,44 was 63 ≥20 : 1

Open Access Article. Published on 30 January 2014. Downloaded 9/30/2021 8:55:31 PM. employed in the reduction step and i-PrOH was employed as the sacrificial hydrogen source.45 Primary alcohols were obtained under the optimized conditions with high selectivity. 83 ≥20 : 1 A wide range of functional groups was tolerated by the system. Despite the diﬃculty in functionalizing the aliphatic olefin, it is encouraging to see that even the aliphatic olefin is able to 74 ≥20 : 1 provide hydration products, albeit at reverse selectivity (Table 11). The role of each reactant was studied by a series of control 56 (1°OH : 2°OH = 1 : 1.4) experiments with styrene as the substrate (Table 12).25 The absence of Pd catalyst shut down the oxidation process totally 54 (1°OH : 2°OH = 1 : 1.9) and only aﬀorded the over-reduction product, ethylbenzene, in ’ a 26% yield. Without Shvo s catalyst, it is observed that no 12 (1°OH : 2°OH = 1 : 2.1) alcohol products were formed and the aldehyde was the major

product. CuCl2 originally serving as a co-oxidant, appeared to be critical in slowing down the over-reduction process. In the

absence of CuCl2, substantial quantities of ethylbenzene were formed. The exclusive formation of aldehyde in the absence of i-PrOH illustrated its role as the reductant. The critical role of 1,4-Benzoquinone (BQ)46 in the reactivity and selectivity of this carried out without BQ. No oxygenated product was formed catalytic system was shown in the lower yield and regioselectiv- under anhydrous conditions by removal of water from the reac- ity of the primary alcohol obtained, when the reaction was tion using 4 Å molecular sieves.

Perspective Dalton Transactions

Table 12 Control experiments25

Byproducts (%)

Change Conversion (%) Yield (%)

None 89 77 1.3 0.7 1.5 1.4 No PdCl2(CH2CN)2 34 0 26 0 0 0 No Shvo’s catalyst 80 0 0.2 0 42 0 No CuCl2 >99 48 32 5.9 0.5 1.4 No BQ 58 0 0.9 0 0 0 No i-PrOH 88 0 0.5 0 57 0 No t-BuOH 48 18 2.0 Trace Trace Trace No t-BuOH, but 28 equiv. H2O 75 64 2.0 4.9 0.96 9.9 Replace H2O with 4 Å molecular sieves >99 0 57 0 0 0

Creative Commons Attribution 3.0 Unported Licence. Scheme 17 Anti-Markovnikov oleﬁn hydration by a triple-relay catalysis system. Scheme 18 Tandem hydroformylation/hydrogenation.

The mechanism of this triple-relay catalyst system is pro- mediated by just Shvo’s catalyst or cyclopentadienone-ligated posed as three tandem reaction cycles as shown in Scheme 17. II Ru tricarbonyl complexes alone (Scheme 18). In the presence of t-BuOH, the olefin undergoes Pd -catalyzed This system reported by Nozaki et al. oﬀers an alternative oxidation to form a linear t-butyl vinyl ether, which is preferred pathway to the tandem synthesis of linear alcohols. It was due to the steric bulk of the t-butyl group. It is noteworthy that

This article is licensed under a reported that Shvo’s catalyst was more active than the conven- this t-BuOH nucleophilic attack plays a key role in the high 6,43b tional Ru hydrogenation catalyst due to its robustness under anti-Markovnikov selectivity. Subsequently, the generation CO pressure, which is insightful for selective hydrogenation of acids (HCl and hydroquinone) during the oxidation process catalyst design for more attractive industrial applications.

Open Access Article. Published on 30 January 2014. Downloaded 9/30/2021 8:55:31 PM. converts the vinyl ether to an aldehyde in the presence of Other tandem catalytic sequences of branched olefins were water through acid-catalyzed hydrolysis. Finally, the primary also carried out for alcohol formation. One sustainable alcohol is formed via Ru-catalyzed transfer-hydrogenation of process that can be tailored to the production of plasticizer the aldehyde. alcohols was reported by Harvey et al.49 The approach begins With the high catalyst loadings and use of stoichiometric with oxidation of 2-ethyl-1-hexene to a diol followed by a BQ, this approach is far from perfect. Nevertheless, it is still dehydration and subsequent hydrogenation to yield an anti- highly encouraging that promising results have been obtained Markovnikov product (Scheme 19). with non-activated olefins such as styrenes and 1-octene using This process generally complements the oxidation/ this eﬃcient tandem catalytic system. This process is an attrac- reduction tandem strategy for alcohol formation especially tive strategy to realize complex structural transformation that from branched olefins. However, the system is rather complicated leads to potential industrial applications, as highlighted by Hintermann more recently.47 Along with this Wacker oxidation/hydrogenation approach, tandem hydroformylation/hydrogenation simultaneously attracted investigation albeit the homologation eﬀect in the process. Nozaki et al. reported a one-pot regioselective olefin hydroformylation/hydrogenation using Rh/Ru (Shvo’s catalyst) dual catalyst system.48 The success of this system was attribu- ted to the orthogonal relationship between each catalyst and that the Shvo’s catalyst is relatively inert in the hydroformylation step. Furthermore, it was found that the regio- Scheme 19 Oxidation/dehydration/hydrogenation route to an anti- selective hydroformylation/hydrogenation of 1-decene could be Markovnikov product.

Dalton Transactions Perspective

Acknowledgements

This contribution was financially supported by the National University of Singapore and A*STAR through generous research grants (R143-000-523-133 and R143-000-535-305). J. Guo is grateful to A*STAR SERC for a postdoctoral grant (PSF R143- Scheme 20 Anti-Markovnikov reductive hydration of alkynes. 000-535-305).

and harsh conditions involving the usage of peracetic acid for oxidation are required. Notes and references The tandem catalytic strategy was further extended to 126 the regioselective hydration of alkynes that retain many of 1 D. B. Grotjahn and D. A. Lev, J. Am. Chem. Soc., 2004, , the characteristics of alkenes. Herzon and Li reported a 12232. Ru-catalyzed anti-Markovnikov hydration system from alkynes 2 M. Tokunaga and Y. Wakatsuki, Angew. Chem., Int. Ed., 37 (Scheme 20).50 This strategy provides yet another route 1998, , 2867. to linear alcohol formation from unsaturated aliphatic 3 L. Hintermann and A. Labonne, Synthesis, 2007, 1121. ’ compounds. 4 M. B. Smith and J. March, March s advanced organic chemistry, John Wiley and Sons, New York, 2001. 5 M. Beller, J. Seayad, A. Tillack and H. Jiao, Angew. Chem., Int. Ed., 2004, 43, 3368. 4. Perspectives 6 J. Muzart, Tetrahedron, 2007, 63, 7505.

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